Photoresist film, silicone oxide film or the like, when deposited on the substrate of a photomask or wafer for IC use generally forms an extremely thin transparent film having a thickness approximately 400 .ANG. to 30,000 .ANG.. The typical line width of an IC pattern obtained thereon through exposure development, etching, etc., is approximately 0.3 to 100 .mu.m.
For proper production control of ICs, therefore, a film thickness measuring device is needed with which such film thickness can be measured accurately with high repeatability. In known devices for measuring film thickness there has been commonly used a method in which measurement is made of the diffracted reflection spectra for a large number of reference specimens. The reference values are stored in the memory of a signal processing device such as a micro-computer, and then measurement is taken of the diffracted light reflection spectrum of the specimen to be measured to thereby determine the specimen film thickness by the ratio of its spectral photo-intensity to the reference specimen. FIG. 1 depicts prior art apparatus of this type. In FIG. 1, specimen 2 is set on the specimen mount 3 of microscope 1 and a white light source 5 illuminates the specimen through the object lens 4. Eyeglass 6 is for observing the surface of the specimen 2 as well as focal point adjustment 7, which has a small aperture for allowing through passage of light reflected from specimen 2. A concave diffraction grating 8 generates a diffracted spectrum, and sweeping device 9 sweeps a given wavelength range, e.g. 400-800 nm, with the concave diffraction grating 8 turning in the direction of the arrows A . 9a is a wavelength counter, 10 a stepping motor, 11 a slit, 12 a photomultiplier tube, and 13 a current-voltage converter with built-in amplifier. Micro-computer 14 controls a stepping motor 10 and also does statistical processing of photo intensity signal K from the current-voltage converter 13 for computing the film thickness and outputting the result, as necessary, to the CRT 15a or to an output device 15 of a printer 15b. 14a is the keyboard of the computer 14.
With this known film thickness measuring device, the film thickness is measured as follows: with the specimen 2 placed on specimen mount 3 of the microscope, the measuring device is started with the intersecting point of two marking lines matching a measuring point P, best seen in FIG. 7, of the specimen 2, confirming this through the eyeglass 6. The reflected light from specimen 2 enters incidently into the concave diffraction grating 8 through the aforesaid optical system of microscope 1. This concave diffraction grating generates a diffracted spectrum corresponding to any given wavelength in a predetermined range, e.g. 400-800 nm, the grating being rotatably driven via sweeping device 9 by stepping motor 10. This diffracted spectrum enters through a slit 11 into a fixed photomultiplier tube 12 and generates a serial spectrum signal I synchronous with the sweeping by the concave diffraction grating 8. Then a photo-intensity signal K is obtained that corresponds to the reflection spectrum multiplied by the current-voltage converter 13. This photo-intensity signal K is processed by the micro-computer 14 for computation of film thickness, and the result is displayed on the output device 15 or printed out. Prior to measurement of specimen 2, measurement is made of the diffracted reflected spectra of a multiplicity of reference specimens of known film thicknesses, and the data for these are stored in the microcomputer 14, as mentioned above, to serve as reference data. In the prior art technique, in which the spectral signals are obtained from the photomultiplier tube serially and synchronously with sweeping by the concave diffraction grating, the diffracted spectral signals are outputted successively during sweeping. Hence there is a limitation due to shortness of the measuring time, this being particularly the case when the sweeping is done a plurality of times for more measured data. Furthermore, during this sweeping, the device is subject to external disturbances, e.g., variations in the intensity of light source due to mechanical vibration, and this may reduce the measurement precision. The present invention is aimed at providing a film thickness measuring device free of such problems.